In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams.
However, it was demonstrated in practice that the membrane performance can deteriorate very quickly without the use of a pretreatment system. The major reason for the loss of membrane performance is heavy hydrocarbon liquid condensation on the membrane surface. Condensation can be prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a pretreatment regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons ranging from C6 to C35 from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes.
Although these pretreatment systems can effectively remove heavy hydrocarbons from natural gas streams to control the dew point of natural gas, the cost is very high. Some commercial membrane projects showed that the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the pretreatment system cost or total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. On the other hand, in recent years, more and more membrane systems have been applied to large offshore natural gas upgrading projects. For offshore projects, the footprint is a big constraint. Hence, reduction of footprint is very important for offshore projects. The footprint of the pretreatment system is also very high at more than 10-50% of the footprint of the whole membrane system. Therefore, the removal of the pretreatment system from the membrane system will remarkably reduce the cost and footprint of the membrane system especially for offshore natural gas applications such as floating production storage and offloading vessel (FPSO) applications.
Conditioned natural gas has been used as fuel gas in gas engines and turbines in the hydrocarbon processing industry particularly for offshore platforms and remote locations and will be used in future floating liquefied natural gas (FLNG) and FPSO applications. To improve the reliability and reduce unscheduled downtime of the equipment that is used for fuel gas conditioning, a simple fuel gas conditioning technology is required. Rubbery polymeric membranes that can selectively and efficiently permeate heavy hydrocarbons and other contaminants such as CO2, H2S, and water vapor will allow conditioning of fuel gas.
A rubbery polymeric membrane that can selectively permeate condensable heavy hydrocarbon vapors such as C3+ hydrocarbons and can reject non-condensable gases such as methane can also be used for natural gas liquid (NGL) recovery.
The present invention describes a new type of nanoporous macrocycle-containing cross-linked polymeric membranes. The introduction of nanoporous macrocycles such as α-, β-, and γ-cyclodextrins (CDs) to rubbery polymeric membranes in the present invention significantly improves both permeance and selectivity of the rubbery polymeric membranes for separations.